"I was such a fan that I modelled my own signature on his, because we have the same initials," recalls Capaldi, who turned 60 last week. "He had a rather florid P, which I copied."
Before the connoisseurs of collectables start calculating the coin such a trove might fetch, Capaldi has some bad news. In his late teens, as he headed off to art school and turned his attentions to "sex and drugs and rock'n'roll" (he was briefly the lead singer of a punk band called The Bastards from Hell), he felt it was time to put aside such childish things.
"I didn't want to be a geek any more so I threw out all my geeky stuff. I had a sort of bonfire of the vanities where I got rid of all this stuff that would be great fun for me to have now. Isn't that terrible?"
Well yes, but it's possibly what makes him so open to meeting today's fans. "I suppose I can put myself in their shoes," he concedes. "I guess I feel their pain."
Though he's taken the past year off to refuel – "Doctor Who is great, but it's a very 24/7 kind of job, and I'd been consumed by that for four years" – he's looking forward to getting back to work, and adding to the insanely long list of credits he's racked up since his screen debut in 1982.
Perhaps the most famous of those credits, apart from a certain Time Lord, is the foul-mouthed political spin doctor in the British TV series The Thick of It and the movie In the Loop. For a (presumably) different crowd, Malcolm Tucker is almost as much a cult figure as is the Doctor.
"I love Malcolm, and the further away I get from him in time, the more warmly I feel about him," says Capaldi. "He was an extraordinary creation, much more of a performance for me than Doctor Who is.
"I'm a humble human being and Doctor Who is a Time Lord, so there's a great leap of imagination," he explains. "But he doesn't use as much bad language as Malcolm does, and he's not as angry as Malcolm is. With Malcolm I had to reach out for these other aspects that are not natural to me – although some of my friends would disagree."
Malcolm sprang forth from the mind of Armando Ianucci, who also created Veep and the film The Death of Stalin, and Capaldi says the pair plan to work together again. Would that be on Ianucci's film of Charles Dickens' David Copperfield perhaps?
"I can't say," he says, chuckling and looking across at his agent, who confirms that, indeed, he cannot say.
One thing he can say is that he would quite like to direct again. And why not? Though he doesn't trumpet the fact, Capaldi has an Oscar to his name, for his 1993 short Franz Kafka's It's a Wonderful Life.
Starring Richard E. Grant as the writer struggling to craft the perfect opening line for his novel Metamorphosis, the darkly surreal comedy was his first stab at directing (he also wrote it). And you get the sense Capaldi is ever so slightly embarrassed by his beginner's luck.
"I didn't really want to be a director, I just wanted to have a go at seeing what it was like," he says.
"I didn't know they gave Oscars for short films. It just kind of threw a rather lovely spanner in the works."
Though, on reflection, it was probably a sonic screwdriver.
Supanova is at the Melbourne Showgrounds April 21-22, and the Gold Coast Convention & Exhibition Centre April 28-29. Details: supanova.com.au
Karl has been a journalist at Fairfax Media since 1999, in a variety of writing and editing roles. Karl writes about popular culture with a particular focus on film and television.
Morning & Afternoon Newsletter